1. Field of the Invention
This invention relates to a spot printing method in a rotary press with a spot printing function which has a plate cylinder having fitted thereto a plurality of plates, and to a blanket cylinder for spot printing.
2. Description of the Prior Art
In an offset rotary press with a spot printing function having a plate cylinder to which a plurality of plates are fitted, a pair of blanket cylinders are disposed to face each other and blanket cylinders for spot printing are fitted to one of the blanket cylinders at its upper part, for example. Each plate is fitted to each blanket cylinder.
A paper web is first clamped between the pair of rotating blanket cylinders and printing is made simultaneously on both surface of the paper web. While keeping contact with one of the blanket cylinders, the paper web is further interposed by the blanket cylinder for spot printing and spot printing is additionally made to one of its surfaces.
Therefore, in the printing process, the paper web continuously receives twice the printing pressure by the other of the blanket cylinders and the blanket cylinder for spot printing while being wound on one of the pair of blanket cylinders.
The prior art technique relating to fitting of the blanket to the blanket cylinder in a rotary press is described, for example, in Japanese Utility Model Publication No. 10001/1975 and Japanese Utility Model Laid-Open No. 39865/1980.
In the former reference, the blanket is fitted to the blanket cylinder by fitting both ends of the blanket to a take-up shaft disposed rotatably in an axial direction in the proximity of the outer peripheral surface of the blanket cylinder, rotating then both ends of the take-up shaft through worm gears and winding the blanket. The blanket and the take-up shaft are single throughout the entire length of the blanket cylinder, respectively.
In the latter, the blanket is fitted to the blanket cylinder by fitting fillet bars at both ends of the blankets into grooves formed on the peripheral surface of the blanket cylinder in the axial direction, further setting them into a groove formed in the axial direction on the peripheral surface of a tension bar disposed rotatably inside the groove, rotating the tension bar and thus winding the blanket. The blanket and the tension bar are divided into two units at the center of the blanket cylinder, respectively.
Generally, in an offset rotary press with a spot printing function having a plate cylinder to which a plurality of plates are fitted, a phase difference occurs between the surface of a paper web wound to one of the rotating blanket cylinders and the peripheral surface of the blanket cylinder due to minute variation of tension of the paper web, elongation of the paper web, delicate creases, and the like, though the paper web is kept wound on the blanket cylinder. If the printing pressure of the second time due to spot printing is applied in succession to the first printing pressure, so-called "double", wherein characters, patterns, and the like, that are printed on the opposed web surface from the blanket cylinder on which the web is wound become thicker or get doubled vertically or horizontally, is likely to occur.
This "double" invites a remarkable drop of quality of the printed matters and must therefore be avoided by all means.
According to the blanket cylinder equipped with the blanket fastening device in accordance with the prior art, the printing pressure is applied to the full width of the blanket cylinder for spot printing, inclusive of unnecessary zones, even when printing is not necessary for the full width of the blanket cylinder in spot printing (such as when spot printing is made to only part of pages of a plurality of pages in the transverse direction, for example).
Even if a blanket having a half width is selected and fitted independently to the blanket cylinder, though such an arrangement is not disclosed in the afore-mentioned Japanese Utility Model Laid-Open No. 39865/1980, the printing pressure is applied unavoidably to the paper web inclusive of the unnecessary zones in spot printing covering only a 1/4 width (e.g. for one page), for example.